1. Field of the Invention
The invention relates to the field of optical networks for transmitting data conveyed over optical connections in the form of optical signals organized into packets.
The invention may be applied in particular to (dense) wavelength division multiplex ((D)WDM) ring networks.
2. Description of the Related Art
Some networks of the above-mentioned type, for example the Dual Bus Optical Ring Network (DBORN), which is a metropolitan access network, comprise communications stations (also known as nodes) coupled to at least one optical fiber adapted to transmit packets of wavelength division multiplexed optical signals and using a void filling protocol to manage the insertion of packets awaiting transmission stored in one of their memories. One such network was the subject matter of the presentation “DBORN: a shared WDM Ethernet bus architecture for optical packet metropolitan networks” given at the Photonics in Switching PiS'2002 Conference, Cheju Island, Korea, Jul. 2002, TuC3.
In the present context the term “void” means an empty space, i.e. one with no packets, within a burst of packets of optical signals.
The above protocol is based on observing the traffic in the transmission optical fiber corresponding to each wavelength, in combination with the use of a delay fiber line inserted into the optical connection.
Observation is generally effected by means of photodiodes each dedicated to one of the working wavelengths of the station concerned and delivering to a monitoring module detection signals that are generally representative of the durations of the voids.
The delay fiber line between the photodiodes and the senders of the station delays the packets in transit for the time needed to process the detection signals.
The processing of the detection signals by the control module consists in determining the duration of the void that has been detected and the associated wavelength, then determining if that duration allows the insertion of one of the packets awaiting transmission, and, if this is the case, organizing the insertion of that packet by the sending module of the station at the wavelength associated with the detected void.
The delay fiber lines equipping the stations of the network have a length chosen to enable the stations to insert the longest packets that the network is able to process. In an Ethernet network, for example, the longest packets comprise around 1500 bytes. A sliding window is therefore created with a duration corresponding to the maximum length of the packets, and this avoids collisions between packets.
The drawback of that type of void filling protocol is that the length of the delay line imposes a maximum packet insertion time that corresponds to a packet size proportional to the transmission bit rate. Consequently, for a given packet size, the lower the bit rate, the longer the fiber delay lines have to be, which can lead to a significant increase in the overall length of the connection and thus increase the cost of the network.